By using 'standard' glider-logic components such as guns and eaters,
we can build any finite piece of circuitry desired; e.g. one which
would simulate a real computer. But to show that Life is computation-
universal, we need to be able to simulate a Turing machine with its
infinite storage capacity. In "Winning Ways", Conway describes an
infinite storage device. It consists of a block located along a
diagonal; the location of the block represents a nonnegative integer.
Glider salvos are used to push the block (increment the register) and
pull the block (decrement the register). A glider fired across the
diagonal at the zero position is used to test the register to see if
it equals 0. (If so, the block is destroyed by the test and must then
be rebuilt using 2 more gliders.) So it's possible to build a
register which can store any nonnegative integer and which has 3 basic
operations: increment, decrement, and test for zero. Marvin Minsky
showed that just 2 such registers suffice to simulate a universal
Turing machine, but that 3 are far more efficient. (See Sec. 15.1 of
Minsky's "Computation: Finite and Infinite Machines".)
*[Note, however, that for any constant k,
a k-counter machine can be exponentially slower
than an equivalent Turing machine.
For an alternative, see my description of the
extensible delay-line memory. --PBC]*

Conway's design uses a 2-glider salvo to pull the block 3 units and a 30-glider salvo to push it 3 units; of course it has never been built. But we can do much better: After examining a couple thousand collisions between 2 gliders and a block, I found a 2-glider salvo which can pull a block 1 unit and a 3-glider salvo which can push it 1 unit. I then built a shotgun which produces the necessary salvos and a control device which releases them when instructed to do so by external circuitry. This device is not exactly like the one described earlier: there is no separate test for zero. Instead, the device automatically reports the transition from 1 to 0. (If you want to know if the register currently equals 0, just send an increment command followed by a decrement command.) This change greatly simplifies the design, since the test does not destroy the block.

Figure 0 shows how to decrement the register. Gliders R and S are about to pull the block 1 unit; glider Z is destroyed in the process. But if you start with the block farther southeast, then Z will escape unharmed. Thus the absence of Z indicates that the register has been decremented to 0. (The register should not be decremented if it already contains 0; doing so would destroy it.)

.........s..... .......s.s..... ........ss..... ............... ............... ............... ............... .........r...oo ..........rr.oo zz.......rr.... .zz............ z.............. Figure 0

Figure 1 shows the increment salvo. By themselves, gliders P and Q push the block 1 unit but also create a boat and a traffic light; glider R prevents these from forming. There are several possible positions for R; I chose this one because it's on the same flight path as one of the decrement gliders, which simplifies the design of the shotgun.

r.................. .rr................ rr................. ................... ................... ................... ................... ................... ...............q.q. ................qq. ................q.. ................... ................... .................oo .............p...oo ..............pp... .............pp.... Figure 1

The sliding block control unit has 3 parts. At the upstream end is a shotgun which produces, every 120 generations, a 4-glider salvo containing both the increment and decrement salvos. Next are the suppressors: a pentadecathlon and a pair of guns which normally delete all 4 gliders from the shotgun but which can release either an increment or decrement salvo when told to do so by an external signal. Finally, there's the zero-detector, a gun which supplies the glider Z in Figure 0. This glider usually escapes; it's timed so that it misses both the increment and decrement salvos. But when the register is decremented from 1 to 0, Z gets deleted; the external circuitry should detect its absence.

The shotgun consists of 4 p120 guns, 4 p30 guns, 4 buckaroos (the queen bee shuttle used to reflect a glider 90 degrees), and 2 rephaser blocks, as shown in Figure 2. Here "G" is a p120 gun, "g" is a p30 gun, and "R" is a rephaser; buckaroos are shown as bends in the glider paths. Each pair of p30 guns is used as a 180 degree reflector since the spacing won't allow a pair of buckaroos to be used.

G G \ / \/ g /\ \ / \ /\ /\ g G \ X \/ \/ / \ / \ R / / X \/\ / / G / \ /\ X / \ / \/ X \ / \/ /\ / \ \ / / X \ X / / \ X \ / < \ / \ \ /\ \ X \ \ / \ \ / \ \ \ g \ /R\ \ \ \ \/ \ \ \ \ \ \ \ \ g \ \ \ output Figure 2

The purpose of the suppressors is to selectively release increment or decrement salvos from the shotgun. Originally I had planned to use a 5-glider salvo which was a disjoint union of the increment and decrement salvos. One suppressor would have deleted the increment salvos and the other would have deleted the decrement salvos. In order to increment or decrement the register, the external circuitry would delete a glider from one of the suppressors. But with p120 logic, the zero detector would have hit the increment salvo, so I decided to use a non-disjoint union of the salvos instead. This makes the suppressors's job more difficult, since they must delete either 1, 2, or 4 of the gliders, depending on whether the register is to be incremented, decremented, or neither. After much experimenting, I came up with the design shown in Figure 3.

The decrement suppressor is a p120 gun supplying the glider D in Figure 3; the increment suppressor supplies glider I. Normally both D and I are present; they destroy all 4 gliders from the shotgun. If D is deleted, then I destroys only P and Q, allowing a decrement salvo (R and S) to escape. If I is deleted, then D passes behind P and Q, reflects 180 degrees from the pentadecathlon, and deletes S, thereby allowing an increment salvo (P, Q, and R) to escape. (If both D and I are deleted, then the block is first pushed and then destroyed, effectively setting the value of the register to infinity. Conceivably this could be useful.)

...................o. ..................ooo ..................... ..s.................. s.s...............ooo .ss.................. ..................o.o ..................o.o ..................... ..................ooo ..r.................. ...rr................ ..rr..............ooo ...................o. ..................... ..................... ..................... ..................... .................q.q. ..................qq. ..................q.. ..................... ..................... ..................... ...............p..... ................pp... ...............pp...i .........dd.......ii. ..........dd.......ii .........d........... Figure 3

Figure 4 shows a schematic of the unit, as it would normally be used.

input input output (decrement) (increment) / \ \ Increment / \ \ suppressor / \ \ / / \ \ / / \ \ / / \ Shotgun \/ / \ \\ / / \ \\ PD / / \ \\/ / / \ X\/ / \ / \\ / \/ \\ / / \\ / / \\/ Decrement X\ suppressor / \\ Zero \\ detector \\ Block Figure 4

To test and demonstrate the unit, I built a simple external circuit consisting of 2 p30 guns, an eater, and a kickback glider. (To decrease the size of the pattern, I put the extras to the southeast. Normally the external circuitry would be to the northwest to stay away from the block's diagonal. I also positioned the increment suppressor fairly close to the diagonal; it would normally be farther away to allow access to its output from northwest.) The block is initially in position 3 and gets incremented to 4. From then on, the output of the zero-detector is inverted twice and becomes the increment suppressing glider, so the increment salvo will be released only when the register is decremented to 0. Meanwhile, a glider is kicked back and forth between the decrement suppressor's stream and an external gun, deleting one glider from the decrement suppressor every 480 generations. So the register gets incremented once, then repeatedly decremented until it reaches 0, and from then on is alternately incremented to 1 and decremented to 0 forever.

The test pattern's initial population is 1685 and it fits in a 240 by 192 rectangle.

[Note added 11/30/96: The p120 guns used in this pattern are obsolete. I never got around to rebuilding the SBM using smaller guns.]

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